for the most part I don't really have any general mob types that I like in particular, though anything big(rams, malboros, gigas) is fun.
Dragons/Wyrms would probably be my favorite type of mob to fight just bc of their size and majesty
but there are some of specific mobs that I've enjoyed fighting,

Ark Angels: this is one of the funnest bc's i've been a part of. Fighting demonic versions of yourself(Ark Angel GK) is hella fun

Shadow Lord: I've done it multiple times now, and it's not as exciting now, but when I first did this fight, it was incredibly exhilerating.

Behemoth: Just so fun, i've loved behemoths in past ff's and this version is bigger and better than ever.

mobs i hate; anything that does any kind of paralyze or aoe or casts -ga spells...all things that destroy a thief